Vibrations or u-joint wear

[Roy B.]

For you people that are restoring your Corvette C1's, I've had many people ask about the "trans cross mount" support having (shims) and why or how do you determine the angle on a stick or Automatics to get the drive shaft aligned correct so that the U-joints don't bind or get a vibration.Using a bubble gage reading in degrees placed on a flat area in line with the rear pinion read the angle.

The idea is to make them COMPLEMENTARY. So, if the rear is, say, 3 degrees
DOWN bubble, you want to lift the trans to be 3 degrees UP. This way, the
Front and rear U-joint form a parallelogram and will spin together happily
Without strain.
If you could visualize an imaginary line coming straight out of the trans would go down a bit, and one coming out of the pinion up a bit, but the two lines would be PARALLEL to each other, never meeting. Make sense?

When ever replacing rear springs, motor mounts or trans. mount this should be checked .IF you have a vibration and cant locate it I would start there. Hope this may help others out there.

[Terry F.]

Re: Vibrations or u-joint wear

I always wondered about that. Thanks, Terry

[Duke W.]

Hook vs. CV joints

Conventional universal (Hook) joints as used on driveshafts of vintage cars and Corvettes IRS axles from '63 to '96 are different than constant velocity joints.

If the transmission mainshaft is rotating at constant speed and the driveshaft is slightly angled to the transmission mainshaft, say 2 degrees, the driveshaft speed is not constant, but contains a small sinosoidal component (that increases with angle) with a mean rotation speed the same as the transmission mainshaft speed and a frequency equal to driveshaft speed, so the complete sinosoidal cycle is completed in one revolution.

If the input angle to the axle yoke is also 2 degrees, the sinosoidal oscillation of the driveshaft will yield constant rotation speed at the drive pinion. Thus it is best to have equal angles at each end. Other than equal angles at both ends can give rise to unpleasant vibrations.

Hook joints should always be operated at a slight angle. Otherwise the load is concentrated in one spot, which can shorten joint life. A slight angle causes the trunnions/needles to wiggle back and forth once per revolution, which spreads the load. Hook joint angles of one to three degrees are best to insure good longevity.

Constant velocity joints are so called because driveshaft or axle shaft velocity is constant even if there is a slight angle, as above, between the gearbox output shaft and the driveshaft or axle.

Duke

[Roy B.]

Re: Hook vs. CV joints

OK Duke! with all your education can you tell me what you said in simple third grade language ( like 30 words or less) I don't have a BA!

[Michael H.]

Re: Hook vs. CV joints

Duke,

Maybe I'm remembering this incorrectly but, isn't the cycle completed twice during one complete 360* rev of the drive shaft?

[Duke W.]

Re: Hook vs. CV joints

IIRC the cycle is once per revolution - speeds up for have a revolution and slows down the next half.

Duke

[Michael H.]

Re: Hook vs. CV joints

I'm remembering all of this now. Unlike the description of most rotating masses, 180* equals one complete cycle when we're talking about drive/driven shafts that operate through universal joints. In 180*, the entire unit is back to it's original starting configuration, even though both components have revolved 1/2 turn. So, the unit actually goes through the speed cycle change two times during one complete 360* revolution of the shafts. The speed differential of the drive/driven shafts is in the range of 10% at 30*. Quite a difference.

The CV style joint reduces the angle of change by half as it enters the joint, and cancels the motion completely as it exits the assembly. The only part of the assembly that has a speed variation is the center housing, or hub, of the CV joint itself.

Nothing is quite as unique as a universal joint. Very complex math.

[Duke W.]

Re: Hook vs. CV joints

Say you're cruising at 3000 revs in top (1:1) gear. If the driveshaft is at a two degree angle to the transmission mainshaft the drive shaft speed will vary in a sinosoidal pattern from say 3030 to 2970 over one revolution.

If the angle into the axle is also two degrees, the drive pinion will rotate at constant speed, so the speed variance of the driveshaft should not cause any problems.

If the angles are different then some sinosoidal rotational speed variation will be apparent in the axle or transmission. Within reason, the spring damper assembly in the clutch disc hub (or fluid coupling or torque converter in an auto trans) can aborb/damp this vibration (and some cars have external driveline dampers specifically for this purpose), but if the angle difference is too great a noticeable vibration may be apparent to the driver and passengers.

Also, if driveline vibration due to differences in u-joint angles corresponds to a torsional vibration mode of the engine, transmission, or axle, then you have a "resonance" situation, which could lead to something breaking in addition to producing an unpleasant vibration.

There is another interesting problem called "driveshaft critical speed". The driveshaft is a beam and like any beam it has vibration modes - like a 2x12 that is used to span a gap on a construction site. If you walk (or run) across it at the right speed which corresponds to its first bending mode frequency, it can jump up and down fiercely or even break.

A length of tube of constant cross section and wall thickness has a natural bending frequency that can be calculated quite easily with a closed form formula.

A couple of years before I joined Pontiac as an engineer right out of college a test driver at Milford driving a high revving short geared GTO had a driveshaft break and come through the floorboard because it reached critical speed.

The Cosworth Vega has a thicker wall driveshaft than 140 CID Vegas because of the Cosworth's much higher rev range, and the driveshaft was stiffened to increase critical speed beyond the reasonably expected speed it would attain in service.

A few years ago I designed and built an auxillary baffle in my Cosworth Vega's oil pan to cure some pressure fluxuation I was experiencing in track events that was evidence of the pickup sucking air. Being as how I know that the CV produces a 13g 233 Hertz forcing function at 7000 revs (due to the second order unbalanced vertical shaking force inherent to conventional inline fours without balance shafts), I was concerned that the baffle might get into resonance.

Fortunately I have a CV buddy who at the time was doing structural analysis work for an aerospance company using the ANSYS program, so he built a finite element model of the baffle and its welded connection to the pan and submitted it to ANSYS for a modal analysis. There were no vibration modes below about 270 Hertz (my spec was not less than 250), which corresponds to about 8000 revs, so I was confident that my baffle and its weld pattern to the pan would not self destruct due to resonaance. Part of the output is line drawings of several vibration modes, showing the (exaggerated) shape of the baffle's various vibration modes. Neat stuff!

Sometimes it helps to be an engineer and know other engineers who have access to the right tools!

Duke

[Duke W.]

Okay...

...everybody substitute "twice per revolution" in my previous posts that say "once per revolution". Since u-joints have 180 degree symmetry, a half revolution speed cycle makes sense.

I just checked my old machine design textbook to verify and was surprised that it says nothing about universal joints, but somewhere I learned of the sinusoidal speed variation of Hook joints. Sorry I didn't get the details right and thanks for the correction.

Duke

[Duke W.]

(Message Deleted by Poster)

Message Deleted by Poster

[Michael H.]

Re: Ha! We're both wrong!!!

Not exactly. The speed increases in the first 90* and decreases in the second 90*, which would be one complete cycle. That would be like saying that an engine intake valve opening event consists of the valve opening when the entire event would have to also include the valve closing, completing the cycle. Cyclic motion requires that the phase be complete from beginning to end, or back to the original starting position. In 180*, the drive shaft and U joint would be back to it's initial starting position.

[Roy B.]

Re: Ha! Duke & Michael

Now I'm a simple guy with a simple mind so what you are all saying! mean anything as to how to shim a drive shaft that I mentioned ?
I know a helicopter flies on haft the rotor blade area %180 when moving in any direction, BUT .

[Duke W.]

You're right, I'm wrong!

For anyone interested, google on "Hooke joint" and there is a wealth of information including all the math.

If the input shaft speed is constant, the output shaft sinusoidal speed completes one cycle in one-half revolution of the input shaft as Michael said or two complete cycles in one revolution of the input shaft.

Regarding Roy's discussion, the transmission mount height should be shimmed so the angle between the driveshaft and transmission shaft is nearly the same as the angle between the driveshaft and drive pinion axis with the vehicle at normal ride height.

At some point if the angular difference is enough, a vibration may become evident.

Duke

[Michael H.]

Re: Ha! Duke & Michael

Roy,

Actually, the discussion that Duke and I have going has everything to do with shimming a transmission to correct the drive shaft angle. If a drive shaft operates completely in line with the transmission output shaft and also the diff pinion and there is no angle at all, there is no issue with drive shaft speed compared to output shaft speed. However, since there IS an angle at both ends, there is a definite difference in the speed of rotation of these components. While the transmission output shaft will be rotating at a steady fixed RPM, that speed, or RPM, will change when it goes through a universal joint that is operating at an angle. It's a slight difference, but still enough to cause concern. That's why the angle at the differential is supposed to be close to matching the angle at the transmission, only in the opposite way, to cancel the angle of the front of the shaft. As the angle increases, the variation in speed of the driven shaft increases. In the first 90* of rotation, the drive shaft begins to travel faster/farther than the transmission output shaft. At 90*, it reaches it's max variation and from there to 180*, it begins to slow to slightly slower than the speed of the output shaft. Then the process starts all over again.

As long as the angle at BOTH ends of the shaft are near equal(but opposite) the speed is corrected and very minor vibration, or moan, is felt. This is mostly felt on acceleration so the angles should be set to equal while the drive train is under a slight/normal load.

I know it's hard to imaging the speed difference but if you study the motion of a universal joint and the shafts connected to it, you will suddenly understand how it works.

If you had a pair of degree wheels on each component, you could actually see(physically) the difference. Start with zero deg., then rotate the shafts until the driving shaft is at exactly 90*. If you have enough angle in the shafts, the degree wheel on the driven shaft will actually show as much as 93* or 94*. Without the effects of this speed change being canceled at the other end of the drive shaft, there would be a terrible vibration/rumble in the car.

[Terry M.]